The invention relates to a self-propelled harvesting machine such as a combine harvester, a forage harvester, a baler or the like, comprising a header that is adjusted relative to a body of the harvesting machine with the aid of an adjusting mechanism in order to cut and recover crop at a substantially consistent height above the ground even when the ground on which the crop stands and on which the harvesting machine moves is uneven.
In conventional harvesting machine, such as that disclosed in DE 38 07 610 A1, the header or the cutting disc is equipped at both lateral ends thereof with sensing bands. The sensing bands swivel about an axis and are elastically pressed against the ground, and the position of the sensing bands provide information about the distance of the header above the ground contacted by the sending bands, wherein a height sensor is provided in each case for detecting this distance. A right and a left adjusting mechanism support the header at the body. A potentiometer is coupled to each of these adjusting mechanisms, the position of which is dependent on the supporting force exerted upon the header by the respective adjusting mechanism. The supporting force is constant as long as the harvesting machine moves on even terrain.
In reality, the ground on which the harvesting machine moves is more or less uneven. If the ground has a raised area, the dimensions of which are small transversely to the direction of travel, the front edge of the header may impact the raised area without the raised area having been detected by one of the ground sensing bands. The header also may impact the ground if the raised area is so steep that the front edge of the header impacts the raised area before one of the ground sensing bands has reached the raised area and has been deflected thereby.
Whenever the header contacts a raised area on the ground, the supporting force exerted by the adjusting mechanisms on the header is reduced. This reduction is detected with the aid of ground pressure potentiometers disposed on the adjusting mechanisms. A control device receives an actual-value signal, which is additively composed of measurement signals of the height sensors and the ground pressure potentiometer, wherein the control device actuates a hydraulic adjusting mechanism on the basis of this signal. The additive superposition of the signals from height sensors and ground pressure potentiometers makes it possible to track the header in such a way that the raised area on the ground is negotiated even when this raised area on the ground is not detected by the height sensors.
A problem of this conventional harvesting machine, however, is that tilting motions that the harvesting machine undergoes when traveling over uneven terrain also have a considerable effect on the supporting forces detected by the ground pressure potentiometers. But such titling motions are not initiated until the wheels of the harvesting machine roll over the uneven terrain, i.e., when the header has already passed the uneven terrain. If the control unit attempts to counteract the supporting-force fluctuations caused by such a tilting motion, there is a risk that resultant vertical deflections of the header could, in turn, lead to ground contact or, in the most favorable case, to unwanted cutting-height fluctuations once the uneven terrain has been negotiated.
In order to prevent this risk, the reaction rate of the conventional control unit must not be too high. But the lower the reaction rate is, the greater the likelihood is that the header will contact a raised area on the ground. This is because the height of the raised area on the ground could not be tracked in a timely manner even if the raised area on the ground was detected by the ground sensing bands. Therefore, the best that can be achieved by means of the conventional technology is to reach a compromise between the contradictory requirements on stability and high reaction rate.
For that matter, the comparators used in the conventional control circuit have only two starting states and, therefore, the valves that are controlled by the comparators and which supply the actuating mechanisms with hydraulic fluid are either open or closed. When the vehicle approaches a dip in the ground and the ground sensing bands detect an increase in the cutting height, the header must be lowered. To this end, when the control circuit opens valves (via which the hydraulic fluid flows out of the adjusting mechanisms), the result is an extreme reduction in pressure, which is initially indistinguishable at the ground pressure potentiometers from load relief caused by the header impacting the ground. In order to counteract this load relief, the control circuit must raise the header and, as a result, the reaction to the change in cutting height is delayed.